Garbage collection on Android

Runtime, build and Gradle Mid-level Kotlin 2.0+ API 24+ Reviewed 31 Oct 2026 By Best Latest Apps

Spot the bug

Which line allocates on every frame?

class Gauge(context: android.content.Context) : android.view.View(context) {
private val paint = android.graphics.Paint()
var value = 0f
override fun onDraw(canvas: android.graphics.Canvas) {
val rect = android.graphics.RectF(0f, 0f, width * value, height.toFloat())
canvas.drawRect(rect, paint)
}
}

Hint 1 of 1

Which object is created inside onDraw?

Choose an answer, then open Show answer below to check it.

Show answer

Answer: line 5 val rect = android.graphics.RectF(0f, 0f, width * value, height.toFloat())

Line 5 creates a new RectF every time onDraw runs, which can be 60 or more times a second. Keep one RectF as a field and update it with set().

30-second answer

ART frees objects that are no longer reachable from GC roots such as threads, static fields and JNI references. Modern ART uses a concurrent, generational collector, so pauses are short, but allocating lots of short-lived objects in hot paths (onDraw, scroll, tight loops) still costs CPU and can cause jank. Leaks happen when a long-lived root keeps something like an Activity reachable.

Practise saying it · 30 s. No recording, just a timer

Explain garbage collection.

Say your answer out loud, then check which of these points you covered:

Reachability from GC roots
Concurrent, generational ART GC
Allocation churn costs frames
Leaks are reachable objects

Deep dive

A GC root reference chain is what LeakCanary prints.

Avoid allocations in onDraw and per-frame code.

onTrimMemory tells you to release caches under pressure.

The low memory killer stops whole processes; GC can't save a process that holds too much.

Common mistakes

Calling System.gc().
Allocating objects in onDraw.
Thinking Kotlin or GC prevents leaks.

Senior take & follow-ups

Explain reachability, then link GC to leaks and jank.

Follow-up questions

Think of your answer first, then open the hint.

What is a GC root?
Show hint
A starting point such as a thread stack, static field or JNI ref.
Why does allocation churn matter if pauses are short?
Show hint
Collection still uses CPU time that frames need.

Similar on iOS: How ARC works iOS uses reference counting instead of a tracing garbage collector.

Suggest a fix if something here is wrong or out of date.